Hydrogels are hydrophilic polymers that absorb water, and are essentially insoluble in water at physiologic temperature, pH, and ionic strength due to the presence of a three-dimensional polymeric network. The three-dimensional network includes crosslinks between polymer chains of the network, and these crosslinks can be formed by covalent bonds, electrostatic, hydrophobic, or dipole-dipole interactions. The hydrophilicity of the hydrogel materials is in large part due to the presence of hydrophilic groups, including, but not limited to, hydroxyl, carboxyl, acid, amide, sulfonic or phosphonic groups, in some instances, along the polymer backbone, and in other instances, as functional side groups that extend from the polymer backbone. Generally, a hydrogel is a crosslinked polymer that absorbs water to an equilibrium value of at least 10% water. The water-swollen equilibrated state of a hydrogel results from a balance between an osmotic force that drives the water to enter the hydrophilic polymer network, and a cohesive force exerted by the polymer chains in resisting expansion. In some fashion, both the osmotic force and the cohesive force correlates with the type of monomers used to prepare the hydrogel polymeric material and the crosslink density of the polymeric hydrogel material. In general, a person of ordinary skill would expect a greater degree of crosslinking for a given hydrogel polymeric material will result in a corresponding decrease in water content, i.e., at equilibrium, the weight percentage of water absorbed by the polymeric network under physiological conditions relative to its total (dry plus water) weight. Water content (%) is simply {[wet lens (g)−dry lens (g)]/wet lens (g)}×100 at equilibrium.
Hydrogels can be classified as synthetic or natural according to their origin; degradable or stable depending on their stability characteristics, and intelligent or conventional depending on their ability to exhibit significant dimensional changes with variations in pH, temperature or electric field. One class of conventional synthetic hydrogels is prepared by free-radical polymerization of vinyl or (meth)acrylate monomers. Several important classes of monomers are recognized by persons of skill with an interest to prepare hydrogel polymeric materials. There are the neutral monomers, for example, but not limited to, methacrylates and acrylates, e.g., 2-hydroxyethyl methacrylate (HEMA), acrylamide/methacrylamides, e.g., dimethyl acrylamide (DMA), glycerol methacrylate (GMA) and cyclic lactams, e.g., N-vinyl-2-pyrrolidone (NVP). At times, the term N-vinylpyrrolidone is used interchangeably with N-vinyl-2-pyrrolidone, and both chemical terms are well recognized by persons of ordinary skill to mean the same vinyl monomer. Another class of monomers is the ionic or charged (under physiological conditions) monomers, including, but not limited to, methacrylic acid, acrylic acid, methylpropylsulfonic acid and p-styrene sulfonate. Typically, in the making of contact lenses the ionic class of monomer is present at low concentration relative to the neutral class of monomer, but the former can have a dramatic effect on water content of the material. The ionic functionality in a buffered saline environment can significantly increase the water content of a hydrogel. For example, copolymerization of 2% w/w methacrylic acid with HEMA results in a hydrogel possessing a water content of 60% (compared with 38% water content for HEMA alone). As used herein “(meth)” refers to an optional methyl substitution. Thus, a term such as “(meth)acrylate” denotes both methacrylic and acrylic radicals.
Hydrogel materials prepared with vinyl cyclic lactams. e.g., N-vinyl-2-pyrrolidone (NVP) can have relatively high water content, and thus, an acceptable level of oxygen permeability. For example, NVP is often copolymerized with an alkyl acrylate or methacrylate such as methyl methacrylate to provide lens materials that typically have a water content of 50% to 80% by weight. However, such copolymers are difficult to synthesize in a controlled manner because of the difference in their respective rates of polymerization between the N-vinyl groups of NVP and the acryloyl or methacryloyl groups of the alkyl acrylate or methacrylate. During free-radical polymerization, the methacrylate monomers polymerize relatively quickly while the vinyl cyclic lactam monomer polymerize more slowly, and therefore, only small amounts of the two comonomers actually react with the other. What one finds is that the polymer network is essentially an interpenetrating network of poly(vinyl monomer) and poly((meth)acrylate)). The result is often a phase separation and a corresponding decrease in the transparency of the polymeric lens material, or the mechanical properties of the lens material deteriorates as the lens absorbs water.
It is also observed, and not to be overlooked, that in a conventional poly(vinyl monomer) and poly((meth)acrylate)) hydrogel framework a minimum of crosslinking occurs between the two essentially homopolymers. In the absence of a suitable crosslinking agent to link the two dual phase polymers, high levels of extractables and dimensional instability results. There have been attempts to design crosslinking agents that address this technical issue. See, U.S. Pat. No. 5,449,729 (Lai, et al), which discloses the use of a crosslinking agent containing both methacrylate and vinyl carbonate reactivity. However, technical issues such as cost to synthesize, toxic preparatory chemistry as well as the relative instability of the vinyl carbonate functionality has limited the development of this dual reactive crosslink agent.
There have been attempts to prepare high water content hydrogels using two different crosslink agents, i.e., allyl methacrylate (AMA) or divinylethylene urea (DVEU), to incorporate the vinyl (cyclic lactam) monomer into the hydrogel polymer network. The AMA crosslink agent works quite well with monomers systems where a fast polymerizing (meth)acrylate and a slow NVP are used. The technical issue with AMA is that it is too volatile and can volatilize during the thermal cure of the polymer resulting in inconsistent levels of crosslinking from one polymerization to the next. Also, DVEU is not a an optimal crosslinking agent because it possesses the same reactivity on the same molecule, and seems to limit the mobility of the poly(NVP) within the hydrogel framework. For example, as films or lenses are being made, or as water enters the framework, the resulting hydrogel material can exhibit loss of lubricity at the surface of the hydrogel. For application of a contact lens, the loss of lubricity is believed to be detrimental to the sensed comfort a consumer will experience in wearing the lens.
Silicone hydrogels combine the high oxygen permeability of polydimethylsiloxane and the excellent water absorption characteristics of a hydrogel. However, for the application of a contact lens, one well known issue with preparing silicone hydrogels is that silicone based monomers are hydrophobic, and relatively, incompatible in regards to forming a homogeneous polymerization mixture with the hydrophilic monomers present in the mixture. The copolymerization of (meth)acrylate functionalized silicones with hydrophilic monomers generally results in opaque, phase separated materials. Technical approaches to minimize such mix incompatibility can include the use of a solubilizing co-solvent or incorporating hydrophilic groups to the silicone backbone.
The design of a silicone hydrogel involves several important considerations. The development involves not only the design of a material possessing excellent physical properties such as modulus, tear strength, and oxygen permeability, but also the design of a material possessing excellent wetting and lubricity without the use of a secondary plasma treatment to impart wettability. The first silicone hydrogels that were commercially introduced in the mid 1990's utilized a plasma treatment to render the surface wettable. This technique is extremely costly and provides marginal clinical performance. Another approach makes use of hydrophilic molds for casting.
A next generation silicone hydrogel material included the addition of a high molecular weight, hydrophilic polymer directly mixed in with the monomer mix formulation. See, McCabe et al. (U.S. Pat. Nos. 6,822,016 and 7,052,131). McCabe takes a somewhat different approach to incorporating poly(NVP). McCabe describe a process of making a polymeric, ophthalmic lens material from a high molecular weight hydrophilic polymer and a silicone monomer. The McCabe process polymerizes the silicon monomer in the presence of an already formed hydrophilic polymer, e.g., poly(NVP) having a molecular weight of no less than about 100,000 Daltons.
Still another approach relies upon the use of a dual phase or a dual network polymerization. The wetting of the latter hydrogel material is achieved through the selective use of monomers with very different reactivity rates as described above, and is often referred to as dual-phase, dual network, or bi-phase polymerization. It is when two or more free-radical, vinyl monomers with two very different reactivity rates provide for a polymer of essentially two homopolymers. The reactivity of the monomers allows for the fast and complete polymerization of the methacrylate functionality followed by NVP. Through careful control of the polymerization rate and degree of crosslinking, high molecular weight poly vinyl pyrrolidone (PVP) chains embedded within a silicone mesh are created. The PVP chains are essentially free to migrate within the silicone matrix and, and in an aqueous environment, are driven to the surface of the lens resulting in good wetting and lubricity. This has been an important discovery in the ophthalmic filed, and it has allowed for improved wetting of a contact lens without the need for plasma or other complicated surface-treatment processes.
The use of the dual phase polymerization has been described several times in the patent literature. It has been used by various research groups for cast molded silicone hydrogel lenses and was first described in a series of U.S. Pat. Nos. 5,387,662, 5,539,016 and 5,321,108, and later in U.S. Pat. Nos. 7,176,268, 7,074,873. In these systems a fast polymerizing methacrylate based silicone is copolymerized with NVP. Recently, U.S. Pat. No. 7,528,208 describes the dual phase polymerization of a monofunctional silicone with NVP. The technical issue with this material, however, is that the crosslinker used for this system is ineffective in maintaining poly(NVP) within the silicone polymer network. This leads to high extractables and poor dimensional stability. U.S. Pat. No. 9,039,174 describes the use of a dual phase polymerization in which a methacrylate based silicone reacts with NVP resulting in a hydrogel material of reported good wetting and lubricity. It is also reported, that a discrete network of PVP can be seen within a silicone network using SEM.